This invention relates to communication systems in which data is transmitted between two vehicles via spread spectrum radio signals; and more particularly, it relates to methods and circuits in such communication systems that compensate for drift in the frequency of the PN codes within the spread spectrum signals.
When data is transmitted via spread spectrum radio signals, the data in the transmitting vehicle is modulated by a pseudo-random code called a PN code. Then, in the receiving vehicle, the modulated data is multiplied by the PN code in order to recover the original data.
For the above modulation - demodulation process to work properly, the frequency and phase of the PN code in the receiving vehicle must be precisely matched to the frequency and phase of the code in the incoming modulated data. However, for various reasons (which are explained in the detailed description) the frequency and phase of the PN code in the transmitting vehicle changes with time. Consequently, the task of generating a PN code in the receiving vehicle which matches the frequency and phase of the PN code in the received modulated data is made difficult.
Of particular importance with regard to the present invention is the fact that in both the transmitting vehicle and the receiving vehicle, the PN code is generated in part in response to a clock signal from an oscillator. That oscillator has a predetermined nominal frequency; however, the actual frequency of the clock signal will deviate somewhat from the nominal frequency. In turn, this frequency deviation, herein called drift, will result in a mismatch between the PN code generated in the receiving vehicle and the PN code in the received data.
To minimize drift in the prior art, highly accurate atomic oscillators were used instead of crystal oscillators. Two examples of typical atomic oscillators are a Rubidium oscillator and a Caesium beam oscillator. However, the atomic oscillators in comparison to less accurate crystal oscillators are very expensive. For example, the cost of a M3000 Rubidium oscillator from EFRATOM Corporation is about $10,000 whereas the cost of an EMXO crystal oscillator from EFRATOM Corporation is about $1000.
Also, atomic oscillators in comparison to crystal oscillators occupy more space, weigh more, and consume more power. For example, the above Rubidion oscillator is 3.25".times.3.25".times.4.5", weighs 2.7 LBS, and consumes 15 watts; whereas the above crystal oscillator is 1.3".times.1.3".times.1.3", weighs less than 0.1 LBS, and consumes 0.45 watts.
Accordingly, a primary object of the invention is to provide a spread spectrum communication system in which PN codes are generated in response to a relatively inaccurate clock signal from a crystal oscillator, and in which a novel method/circuit is used to compensate for drift in the crystal generated clock signal.